Path::get_root is broken using wp-cli if wordpress installation has a different site and home url, w

[duritong]

Description of issue

Running backupwordpress through wp-cli, on an installation that has a different home and site url, always failed with: "Invalid root path"

Using --root on the cli works around this problem.

Details

I tracked this down to https://github.com/humanmade/backupwordpress/blob/master/classes/class-backupwordpress-wp-cli-command.php#L68-L70

Which ends up in Path::get_home_path through Path::get_root and Path::get_calculated_root. In Path::get_home_path a check is being made, whether the siteurl and home are the same.

https://github.com/humanmade/backupwordpress/blob/master/classes/class-path.php#L110

In a case where the installation has a different home than siteurl, this enters this branch, and then it is tried to set the home_path based on the SCRIPT_FILENAME.

https://github.com/humanmade/backupwordpress/blob/master/classes/class-path.php#L113

However, if backupwordpress is used through wp-cli, the SCRIPT_FILENAME for example is the phar file of wp-cli, (e.g. /usr/libexec/wp-cli/wp-cli.phar) and so everything breaks and home is set to an empty string in the end, meaning root is an empty string and so the above error is fired.

When we pass --root=/foo on the cli to the backup command, everything works, as Path::get_calculated_root returns the manually set root-directory.

As I don't fully understand the reasoning behind the branch in Path::get_home_path and the tricks behind using SCRIPT_FILENAME, I'm not really able to say how it should be fixed and I guess somebody with more experience behind this particular code has to look into it.